http://lists.nongnu.org/archive/html/qemu-devel/2011-02/msg01156.html
I thought there were more discussions on the topic but can only find the
above thread ATM.
Do folks here have objections to increasing the migration speed prior to
core dump? If not, any suggestions for a value? I set the migration
speed to 1G in doCoreDump() prior to calling qemuMigrationToFile() with
following improvement dumping a 4G guest

Why any limit at all? When dumping to file rather than network, I can't
think of any justification for a default limit when targetting a local
drive (a particular installation may have a reasonable limit, especially
if the file involved happens to use a network - think NFS). It seems
like it would make sense to override the qemu defaults and go with
unlimited speed for faster dump and save operations.